I can't post the details because of the proprietary nature of this project, but this remains the most complex project I've ever worked on. It is a large web application written completely in HTML and JavaScript that connects to server-side data processing and storage via AJAX using RESTful-style requests. The backend is Drupal, and the frontend is Javascript using ExtJS 2.0 (plus YUI for the History Manager).

The project was quite a challenge. Traditional web applications are much simpler, because every page is a complete reload. Adding AJAX functionally makes things more complicated, but going 100% AJAX (to the point where the page never completely reloads) is significantly more complicated. However, I developed some pretty solid methods for dealing with page "state", global events, and many other AJAX application challenges. I hope to work on another HTML/JS AJAX application in the future.